1. Field of the Invention
This invention relates generally to digital electronic circuitry for generating a periodic function of an input. In particular this invention relates to digital angle to resolver/synchro converter circuits.
Synchros are typically used to transmit position data. The input position signals, referred to herein as the rotor-stator rotation angle .theta., are applied to the shaft of a synchro transmitter and transmitted over a three-wire system to a synchro receiver, the shaft of which is thereby positioned at the proper angular position .theta.. Although four-wire resolver data transmission systems similar to three-wire synchro data transmission systems are feasible, resolvers are used mainly for trigonometric computation such as the conversion of polar to rectangular coordinates or the rotation of rectangular coordinates. Three-wire synchro format data may be transformed into four-wire resolver format data and vice versa by various known devices, such as "Scott-T" transformers. The resolver is so named because it can resolve an input position into two output voltages which vary as the sine and cosine of the rotor-stator rotation angle .theta.. The output equations of a resolver having one stator winding energized by an AC voltage equal to A sin .omega.t are A sin .omega.t sin .theta. and A sin .omega.t cos .theta., assuming that the rotor-stator transfer functions are equaled to one.
It is particularly convenient to utilize resolvers and synchros in digital electronic circits. It is therefore often necessary to convert between digital format and resolver or synchro format by means of digital angle to resolver/synchro converters.
Octant normalization is a known technique for reducing the complexity of required signal processing. The input angle .theta. is normalized per octant so that the input signal range is normalized from a range of 0.degree..ltoreq..theta..ltoreq.360.degree. to a range of 0.degree..ltoreq..theta..ltoreq.45.degree..
2. Description of the Prior Art
Analog periodic function generators, both mechanical and electrical, are well known in the art. Such devices are relatively complicated, require constant adjustment, are usually relatively large and subject to degradation by the environment. A digital electronic approach is attractive because of the inherent accuracy, stability, small size and dependability of digital circuitry.
The generation of a periodic function, such as the tangent of an angle, can be accomplished by means of a tangent table stored in a memory such as a Read Only Memory or ROM.
Digital to resolver/synchro converters are known which utilize electronic circuitry such as the DSC series of converters described in the Analog Devices, Inc., Product Guide of Jan. 19, 1975, which use sine and cosine multiplication. Similar devices are shown in the 1975 Data Device Corporation catalogue.